Although, in theory, a therapeutic drug is employed to treat only diseased tissue in a patient's body, in practice, conventional drug delivery techniques normally provide the same concentration of the drug in both diseased tissue and healthy tissue throughout the patient's body. Since most conventional techniques do not deliver a drug solely to the diseased tissue, a larger dosage of a particular therapeutic drug must be employed than the dose that would otherwise be required to treat the diseased tissue if the drug were delivered only to the diseased tissue. Further, many therapeutic drugs can adversely affect the healthy tissue of a patient, causing undesired side effects that can even be potentially harmful to healthy tissue. Moreover, the conundrum of causing some harm to healthy tissue, so that diseased tissue can be treated with a therapeutic drug, is particularly acute in therapies employing drugs having low therapeutic indices, such as cytotoxic anticancer drugs. Certain photoreactive drugs, while preferentially absorbed by abnormal tissue, are somewhat toxic to normal tissue. When such drugs are infused into a patient's circulatory system with the intent that they be carried to a treatment site within an organ, the photoreactive agent can adversely impact the normal tissue that they contact, having an undesired affect on the patient's health.
Photoreactive agents are employed when administering photodynamic therapy (PDT). During PDT, light is used to destroy abnormal cells in tumors and pathogenic organisms. Commonly assigned U.S. Pat. No. 5,445,608 discloses several different embodiments for transcutaneously implantable light source probes to administer PDT. Preferably, these probes include a plurality of relatively low intensity light sources, such as light emitting diodes (LEDs). In the above-referenced patent, it is generally contemplated that a probe containing a plurality of light sources is transcutaneously introduced to a desired internal treatment site through a surgical incision and left in place for an extended period of time, so that the light emitted by LEDs or other types of light sources included in the probe can administer PDT to destroy abnormal cells that have absorbed a photoreactive agent. Before administering PDT, the photoreactive agent is infused into the patient's body and ideally, should be absorbed only into the diseased cells at a treatment site that are to be destroyed by the therapy. A photoreactive agent has a characteristic light absorption waveband and reacts when exposed to light within that waveband by destroying any cells that have absorbed it. When a light source producing light having the absorption waveband of a particular photoreactive agent is directed at a treatment site at which cells have preferentially absorbed the agent, those cells will be destroyed by the photodynamic reaction. It is clearly preferable that the exposure of healthy tissue to the photoreactive agent be limited, so that the exposure is confined to diseased tissue within the internal treatment site, e.g., within a tumor. Even if the photoreactive agent is injected directly into a treatment site, some of the photoreactive agent may be carried away from the treatment site, and its toxicity may adversely affect healthy tissue and thus, impact on the health of the patient. Clearly, it would be preferable to target the photoreactive agent within the treatment site and minimize the risk of the substance being carried outside the treatment site by bodily fluids.
Researchers have developed both passive and active drug targeting techniques for concentrating therapeutic drugs at particular treatment sites within the body. Two well-known passive drug targeting techniques for the distribution of therapeutic drugs to diseased portions of the body are referred to by the terms "antigen directed" and "liposome encapsulated," respectively. The antigen directed technique is typically employed to bind antibodies to therapeutic substances, such as anticancer drugs or immunotoxins. This substance with the bound antibodies is injected into the patient's body, so that a significant amount of the antibodies are carried by the patient's circulatory system to diseased tissue throughout the body, where the antibodies concentrate the therapeutic substance. The liposome encapsulated technique coats a therapeutic drug with another drug that is known to collect in a particular organ of the body, such as in the liver or the spleen. After the coated therapeutic drug is injected into the patient's body, a certain substantial percentage of the drug collects in the particular organ that is targeted for treatment. Thus, passive drug targeting techniques employ the body's natural processes to concentrate therapeutic drugs at particular treatment sites within the body.
In contrast, active drug targeting techniques do not rely as much upon the body's natural processes to concentrate a therapeutic drug in a particular portion of the body. Instead, external forces are used to achieve this goal. For example, a medical practitioner can direct a strong external force, such as a magnetic field, at a treatment site in order to attract a magnetic fluid that includes a therapeutic drug. In the prior art, there are two types of magnetic fluids employed for active drug targeting. The first type combines a therapeutic drug with a plurality of metallic particles that are suspended in an aqueous solution, such as saline, to create a magnetic fluid. The second type of magnetic fluid employs metallic magnetically attracted particles that are coated with a polymeric material. The polymeric coating is selected for its ability to bind a therapeutic drug directly to the magnetic particles by adsorption, which is reversible. The coated and bound metallic particles are suspended in a solution, such as saline. When either type of magnetic fluid is injected into the patient's body, it is attracted by the strong magnetic field to the treatment site, thereby concentrating the therapeutic drug at that site.
In the prior art, strong magnetic fields have been directed to a treatment site using electromagnetic coils that are disposed outside of the patient's body. However, there are several inherent problems in using an externally positioned magnetic field generator such as an electromagnet. First, the patient is not ambulatory during the treatment, because the patient must remain motionless in close proximity to the external magnetic field generator while the magnetic fluid carrying the drug is infused, so that the therapeutic substance can be thus concentrated at the treatment site. The size and weight of a typical magnetic field generator capable of producing a magnetic field large enough to attract the magnetic fluid to the treatment site and the need to be coupled to a relatively powerful power source to energize the magnetic field generator precludes a patient from easily carrying the generator about while the desired concentrating action of the magnetic flux occurs. Second, it is extremely difficult to narrowly focus an externally produced magnetic field on diseased tissue that is disposed deep within a patient's body. The deeper within the body that an externally generated magnetic field must extend to reach a treatment site, the greater the amount of healthy tissue that will also be exposed to the magnetic field. Since magnetic fluids are attracted to strong magnet fields, the fluid tends to collect in any tissue where such a field exists. Finally, studies have shown that strong magnetic fields can have a deleterious effect on human tissue by increasing the rate at which free radicals are created in cells; free radicals can damage the cells. Thus, there is a need to provide an implantable magnetic source that can produce a magnetic field at a treatment site within diseased tissue, while limiting the overall exposure of healthy tissue to the magnetic field. In this way, a magnetic fluid that includes a therapeutic drug can be selectively targeted and concentrated in a treatment site within a particular portion of the patient's body, without causing adverse effects to other tissue.